1. Field of the Invention
The invention relates to film laminations of thin fluoropolymer films to polymeric receiver sheets. More particularly, the invention pertains to the production of very thin, transferable fluoropolymer films having improved thermal stability.
2. Description of the Related Art
It is well known in the art to produce single layer and multilayer fluoropolymer films. See, for example, U.S. Pat. Nos. 4,677,017; 4,659,625 and 5,139,878. Many fluoropolymer materials are commonly known for their excellent moisture and vapor barrier properties, and therefore are desirable components of packaging films, particularly lidding films and blister packages. In some applications it is also desirable to orient the films to further increase the properties of the films. For example, for a push through lidding used to package pharmaceuticals, it may be desirable to monoaxially orient the film in order to achieve one direction push through of the product. For high strength lidding, a biaxially oriented film would be desired. Oriented fluoropolymer films have also been shown to exhibit improved moisture barrier properties over non-oriented fluoropolymer films.
Fluoropolymers such as poly(chlorotrifluoroethylene) (PCTFE) are exceptionally difficult to orient due to their extremely fast crystallization rate and thermally induced self-orientation. The fast crystallization rate of PCTFE produces a highly crystalline structure that hinders orientation and actually prevents further orientation beyond a certain point. Its thermally induced self-orientation results in a film which, upon unconstrained heating, self extends in the machine or longitudinally stretched direction and shrinks in the transverse direction. U.S. Pat. No. 4,510,301 discloses oriented films containing a copolymer of 40 to 60 mole percent ethylene and chlorotrifluoroethylene. U.S. Pat. No. 4,519,969 discloses a biaxially stretched film and a method for the manufacture thereof, containing at least 90 mole % of ethylene-tetrafluoroethylene copolymer. Various attempts have also been made to produce a multilayer fluoropolymer film structures with thin layers of fluoropolymer. Most emphasis has focused on the selection of the adhesive materials. U.S. Pat. No. 4,677,017 discloses coextruded multilayer films which include a fluoropolymer and a thermoplastic film which are joined by the use of an adhesive polymer. U.S. Pat. No. 4,659,625 discloses a fluoropolymer multilayer film structure which utilizes a vinyl acetate polymer adhesive layer. U.S. Pat. No. 5,139,878, which is incorporated herein by reference, discloses a fluoropolymer film structure using an adhesive layer of modified polyolefins. However, there is a need in the art for a thin fluoropolymer film suitable for use in multilayered packaging films.
There is also a need in the art to have the capability to make stretchable and transferable thin fluoropolymer films at a low cost using conventional equipment. Current fluoropolymer thin film laminates are limited by laminate processing and the costs associated with handling very thin films, thereby requiring the formation of thick fluoropolymer films on receiver sheets. Specifically, currently known techniques require that fluoropolymer films have a minimum thickness of at least about 50 gauge (0.5 mil; 12.5 μm) due to the limitations of the extrusion and lamination processes. In particular, the high melt temperatures required by coextrusion or extrusion coating processes makes it impossible to introduce a thin fluoropolymer film onto certain polymeric receiver materials, e.g. polyvinyl chloride (PVC). Accordingly, in order to laminate, coextrude or extrusion coat certain fluoropolymer films with PVC it is necessary to use thicker fluoropolymer films. However, in many applications, it is desirable to have a thinner fluoropolymer film, i.e. a fluoropolymer film having a thickness of from about 4 gauge (0.04 mil; 1 μm) to about 40 gauge (0.4 mil; 10.2 μm) thick.
As mentioned above, oriented fluoropolymer films are known to exhibit improved properties, but current processing limitations also limit the extent of orientation that may be conducted on unsupported thin fluoropolymer films. Therefore, there is a need in the art to form fluoropolymer film laminates that have a good moisture barrier property and which have thicknesses of less than about 40 gauge (0.4 mil; 10.2 μm).